Three-dimensional thin film solar cell (3-D TFSC) substrates afford cost, performance, and mechanical strength advantages. Compared to traditional flat solar cells with a similar amount of silicon, 3-D TFSCs have superior mechanical strength, better light trapping, and lower cell processing costs because of their self-aligned nature.
The '887 publication discloses, in one embodiment, a three-dimensional honeycomb structure for solar cell production. This honeycomb structure provides enhanced mechanical strength, allowing for processing of preferably free-standing thin substrates, which for the same silicon volume are otherwise mechanically not robust. Traditionally, flat solar cells need to be bonded to a host substrate (e.g., glass), putting temperature constraints on subsequent processing (thus compromising performance) and adding extra cost. In addition, there is parasitic absorption from both the glass substrate and the adhesive used to bond the silicon with the glass, causing solar cell efficiency to be compromised.
Further, 3-D TFSC substrates are inherently better at trapping light than traditional untextured planar solar cells. Ordinarily, a lot of long-wavelength light from the top surface of an equivalent-silicon-volume two-dimensional structure would escape. In a three-dimensional structure, this light will be trapped in the trenches. Finally, 3-D TFSC substrates reduce downstream process steps, further reducing cost.
Despite all the advantages of 3-D TFSC substrates, even greater mechanical strength, efficiency, and potential cost reductions may still be possible. Typically, a template, having reverse features of the resulting 3-D TFSC substrate, is re-used several times to form many 3-D TFSC substrates. Because of the epitaxial deposition process used to form the 3-D TFSC substrates, the template may in some embodiments have wide trenches. Wide trenches in the template correspond to wide silicon wall thickness in the resulting 3-D TFSC substrate. If unit cell apertures are left the same while increasing silicon wall thickness, a performance reduction results. This is due to the fact that the fractional metal coverage increases (assuming that all the ridges are metalized), since metal contacts the exposed silicon on the ridges. Presence of metal prevents sun light from being coupled to the solar cell as it reflects the light falling on it.
To increase efficiency through a reduction of optical reflection losses due to metallization, unit cell apertures may be increased, resulting in a reduction in fractional metal coverage. However, this tradeoff reduces the effective mechanical strength of the 3-D TFSC substrate.
Known methods and systems lack a powerful mechanism for achieving easily implementable tradeoffs between mechanical strength, efficiency, cost reduction, and fractional metal coverage on the solar cell.